1. FIELD OF THE INVENTION
The invention relates to well tools used in the completion of oil and gas wells and more specifically to locking apparatus for positioning well tools, such as safety valves or blanking plugs in nipples within subterranean well conduits, such as tubing strings.
2. DESCRIPTION OF THE PRIOR ART
A common method of positioning a subterranean well tool at a desired position within a well conduit, such as tubing string, is to use a locking apparatus attached to the well tool to anchor the tool in the well. Normally the lock is attached to the top of the well tool and a wireline running tool is attached to the lock to lower the assembly into the well. The lock will normally have an outwardly projecting no-go shoulder for engaging a cooperable no-go shoulder on the nipple to position the lock adjacent to an annular recess defined in the nipple. Outwardly expandable members, such as collets or discrete locking dogs, are employed to engage the annular recess within the nipple. Normally these locking dogs will secure the lock against upward movement while engagement of the no-go shoulders will prevent downward movement of the well tool. The radially expandable members on conventional locking apparatus can then be expanded outwardly by means of springs, by shifting one member to release inherently radially flexible collets, or by axially shifting an inner mandrel beneath the collets or locking dogs to force the radially expandable members outward into engagement with the nipple recess. Interlocking means, such as an enlarged diameter surface on the inner mandrel, can be used to hold the radially expandable members or locking dogs in engaged position.
U.S. Pat. No. 4,510,995 discloses a typical locking tool and is characterized by having locking dogs which carry both upwardly and downwardly directed forces upon full radial expansion thereof into the nipple recess. The expansion of the locking dogs concurrently effects disengagement of the no-go shoulders so that the entire forces subsequently exerted on the lock, either in an upward or downward direction, are absorbed by the engagement of the locking dogs with the nipple recess.
Said U.S. Pat. No. 4,510,995 also discloses a conventional running tool for cooperation with the lock to effect the run-in of the lock to a desired position in the well and the setting of the lock by manipulation of the wireline upon which the running tool is carried. Such running tool relied primarily on shear pins for effecting the necessary interengagement of the running tool with the lock structure and such shear pins were subject to inadvertent breakage whenever obstructions were encountered during the run-in process. Furthermore, the conventional running tool disclosed in said U.S. Pat. No. 4,510,995 has the disadvantage that it is possible, under some circumstances, for the running tool to be actuated by downward jarring to presumably radially expand the locking dogs into engagement with the nipple recess, but the locking dogs are not fully engaged in such recess. Under such conditions, the retrieval of the running tool can be effected but the absence of a complete lock is indicated by the absence of a depending prong on the running tool when it is withdrawn from the well. The fact that an indication of improper setting of the locking dogs has occurred does not really resolve the problem, because a wireline fishing tool has to be lowered into the well to retrieve the partially set locking mechanism and the entire operation then repeated.
The prior art has not provided a running tool which can be retrieved from the well after being manipulated through the lock setting operation without concurrently retrieving the locking tool.